Rotating element displays (REDs) are electro-optic displays that employ rotation of optically and electrically anisotropic elements to generate images. The first REDs were developed in the 1970s at Xerox's Palo Alto Research Center. These displays, often referred to as “gyricon” displays, rotating element displays, or twisting ball displays, are typically based on rotation of optically and electrically anisotropic spheres embedded in an elastomer. In one example of a rotating element display, each sphere is composed of negatively charged black wax or plastic on one side and positively charged white wax or plastic on the other side. Each sphere is suspended in a dielectric fluid contained within a cavity formed in a plasticized elastomer and is free to rotate in the fluid so that it could turn with black or white side to the viewer, thus providing a pixel with a black or white appearance. When an appropriate voltage is applied to the electrodes addressing selected spheres, the spheres rotate in accordance with their dipole moment and display an image to the viewer. Such devices have been described, for example, in U.S. Pat. Nos. 4,126,854 and 6,441,946 issued to Sheridon et al.
Rotating element displays are often referred to as electronic paper, because they can be fabricated in flexible form and can be used without backlight, just as ordinary paper. Rotating element technology, however, failed to produce image quality comparable to that of images printed on paper. In particular, such displays did not possess the high reflectance of white paper, therefore providing low-contrast images. Rotating element displays based on elastomer substrate also had limited environmental stability, because plasticized polymer was not capable of withstanding high-temperature or high-humidity conditions. Further, only few dielectric fluids were suitable for use in gyricon displays, since dielectric fluid in gyricon was serving both as a polymer plasticizer and as a rotation media and therefore had to possess properties suitable for both of these applications.
Rotating element displays are generally limited to black-and-white devices. While use of color filters in combination with gyricons was described, e.g., in U.S. Pat. No. 7,071,895 issued to Wampler, and in U.S. Pat. No. 5,815,306 issued to Sheridon et al., commercial use of such displays was limited due to manufacturing difficulties and due to low brightness and contrast of the resulting devices. Since black and white electrophoretic displays in general and gyricon displays in particular have limited brightness in the white state, adding a color filter on top of the display reduces the brightness of the display even further, creating a display with washed out colors and poor readability.
Other techniques for fabrication of color gyricon displays included the use of multi-segmented multi-colored spheres described in U.S. Pat. No. 5,760,761 issued to Sheridon et al., and the use of multi-layer gyricon stacks, where each layer of the stack is responsible for a specific color, as described in U.S. Pat. No. 6,162,321 issued to Silverman, U.S. Pat. No. 5,767,826 and U.S. Pat. No. 5,982,346 issued to Sheridon et al. The displays described in these references similarly suffered from manufacturing difficulties and were not successfully commercialized. Rotating element devices disclosed in these patents involve particles disposed in an elastomer substrate swelled by a plasticizer fluid, where the particles are disposed in random configuration resulting in poor packing density, lower brightness, contrast and saturation of colors in a device. In addition, the devices described in these patents require complex manufacturing of small multi-segment rotating elements, combining transparent and colored regions.